Woohoo, looks like normal paper-reading services have been resumed...
Disclaimer : I work with the lead author on an unrelated project.
The problem of whether ultra-diffuse galaxies (UDGs) are tiny but massive or just tiny and boring has never really gone away. So far it seems they're all over the shop. Some appear to be such lightweights that lack any dark matter at all, and it's hard to see how they could ever from. Others appear of the opposite extreme, being so massive and dark matter-dominated that they might not fit with standard theories of galaxy evolution. Most seem to be somewhere in the middle, but those extreme values are significant.
The difficulty is that to properly weigh a galaxy's total mass, you need to know how fast it's rotating : there are some clever alternatives, but none are really as good as direct measurements of the kinematics. There are only a handful of cases so far where this has been possible, so every new measurement helps.
This paper presents the atomic neutral hydrogen (HI) measurements for two UDGs in different environments. This is important, since in some scenarios UDGs are formed through environmental processes. The fact that some UDGs (such as one in this paper) are really quite isolated doesn't mean that the majority of UDGS, which are thus far found in clusters, couldn't be the result of some cluster-based process, though it would be a bit contrived and anti-Occam.
The paper is very dense but it's crammed full of science and not excessive details of the observational methods. Their main result is that both galaxies are probably dark matter dominated. Though the resolution isn't exactly exquisite, it's more than enough to see that both objects have ordered rotation.
Of course, there are caveats. The first galaxy has rather messy HI and might have interacted with something. This makes converting its measured velocity width to its true rotation speed more difficult. Their estimates range about 50 - 150 km/s, which means it's either lacking dark matter or highly dominated by it. But the latter estimate, they say, is probably more likely, and its dark matter content in that case (although high) would be consistent with other objects of similar baryonic mass.
The second galaxy is more robust. This is clearly close to edge-on, making the velocity correction smaller and less prone to errors. And this one fits exactly where you'd expect to find a regular dwarf in terms of baryonic and dark mass. But it too has an extension indicating an interaction of some kind - most probably the gobbling up of a smaller satellite, they say. It also has an extremely high ratio of HI to stellar mass. Neither of these features, however, is much of a problem for the kinematic measurements.
So what of those other UDGs that lack dark matter ? Their plot of baryonic mass as a function of dynamical mass is... unclear. If I have to describe it somehow, I'd say there are two distinct populations. One, of normal dwarf galaxies, shows a broad decline in baryonic mass with increasing dark mass. The other, the weirdo UDGs, is a quite distinct cloud. But it's not at all clear - discerning a pattern here is like being given a join-the-dots puzzle without any numbers and trying to work out if it's supposed to be Jesus or a dinosaur. I think the answer is only going to become clear with more and better data. So my only conclusion is that I'm sitting firmly on the fence as to whether these objects are truly strange or just difficult to measure.
Resolved HI in two ultra-diffuse galaxies from contrasting non-cluster environments
We report on the first resolved HI observations of two blue ultra-diffuse galaxies (UDGs)using the Giant Metrewave Radio Telescope (GMRT). These observations add to the sofar limited number of UDGs with resolved HI data. Within the limits of the observations' resolution, our analysis indicates that SdI-2 is dark matter-dominated within its HI radius and this is also likely to be the case for UDG-B1.
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